1. Field of the Invention
The present invention relates to a device for purging evaporated fuel captured by a charcoal canister for an internal combustion engine:
2. Description of the Prior Art
Fuel evaporated from a fuel tank or a float chamber of a carburetor of an internal combustion engine is usually captured by charcoal in a charcoal canister and is purged into an intake path of the internal combustion engine. To improve the fuel capturing ability of the charcoal canister, a large flow volume of purging air is required. However, when the flow volume of the purging air is increased, the air/fuel ratio of the intake gas taken into the engine becomes lean, because the purging gas is introduced into an intake path of the engine downstream of the carburetor. Especially when the flow volume of the engine intake gas is small, such as at low engine loads, the lean air/fuel mixture intake gas impairs the drivability of the engine.
To remedy the above problem, Japanese Utility Model Publication SHO No. 55-170463 discloses improved evaporated fuel purging device. The device is provided with a fixed area throttle which, in one embodiment, always allows a small volume flow of a purge gas including evaporated fuel and two vacuum-actuated flow control valves which are arranged one in series and one in parallel with the fixed throttle. The series flow control valve opens when engine loads reach a predetermined value which is set at a comparatively low engine load. The opening degree of the valve in parallel with the fixed throttle gradually increases in proportion to the increase in the engine load, thereby increasing the purging of the evaporated fuel in proportion to the increase in the intake gas taken into the engine. The flow control valve in parallel with the fixed throttle in one embodiment is controlled by utilizing the vacuum in an intake manifold provided downstream of a throttle valve.
However, the above-mentioned conventional purging device has a very complicated structure and in one embodiment requires two vacuum-actuated flow control valves, resulting in an inevitable increase in cost. In addition, since the device utilizes vacuum in the intake manifold, which usually varies according to engine loads, at engine start when not enough evaporated fuel has yet been captured by a charcoal canister, and when the throttle valve is opened, which usually results in a sudden decrease of vacuum in the engine intake path and a sudden increase in volume flow of the purging gas, the engine is liable to stall, and also control of the air/fuel ratio of the gas mixture is liable to deteriorate. Further, since such a sudden change of the air/fuel ratio can not be fed-back, NOx in the engine exhaust gas is liable to increase and the catalytic convertor can not operate normally